The present invention relates to liquid enamel compositions formed without using solvent. The liquid enamel compositions comprise polyester resins having low viscosity. The liquid enamel compositions of the present invention find utility as solventless coatings or extra high-solids coatings having significantly reduced volatile organic compounds (VOC) content.
A primary component of a coating composition is the film forming component which provides for protection of the substrate and optionally provides a decorative coating. Film formation is the conversion of a coatings composition in a container into an adherent, durable coating. For almost all methods of application, an ideal coating is one that is free flowing during application at the fluidity best suited for the particular method of application but, which once applied to a surface, spreads out into a uniform film, loses its ability to flow and thus forms a film that is smooth and of uniform thickness. This has been achieved in the past by proper selection of a solvent that imparts good viscosity and flow during application of the coating and proper evaporation rate to permit rapid build up of film viscosity, but not so rapid as to result in an undesirably uneven surface. An additional important consideration concerning a coating is the film clarity. Film clarity is particularly important for decorative coatings and in top coat applications.
As a result of environmental concerns and governmental regulations, in particular on the state level, intense research efforts have been made in the coatings industry to reduce the emission of volatile organic compounds (VOC). High-solids coatings, waterborne coatings, powder coatings and radiation-cure coatings are the emerging technologies developed in the industry to meet this need. Among the above technologies, high-solids coatings have been widely accepted since they do not require the conversion of existing conventional manufacturing facilities for implementation of the new technology.
High-solids coatings typically contain 50-70 wt. % of solid resins in suitable organic solvents. Coatings with higher than 70 wt. % solids are more difficult to obtain due to the increasingly high viscosity of the formulation. Efforts are, therefore, being directed to synthesizing resins with reduced viscosity. One way to achieve reduced viscosity is to reduce the molecular weight of the coatings vehicle, normally a polyester resin. However, as the molecular weight of the polyester resin decreases, the resin is more likely to crystallize or form a hazy resin, thus making the resin unsuitable for use in liquid coatings applications. This is especially true if the polyester has a linear structure. It has therefore become a challenge to attempt to design and synthesize a resin suitable for solventless or extra high-solids coatings applications while maintaining good quality and clarity of the formulation. The term "extra high-solids" is used herein to denote less than 20 wt % solvent in the formulation.
U.S. Pat. No. 3,994,851 describes a coating composition formulated from an ester-containing polyol having a molecular weight of less than about 850 and a viscosity of less than about 60 poises, and an amine-aldehyde crosslinking agent. The ester-containing polyol comprises an aliphatic non-alicyclic acid and a polyol. However, the patent does not teach the use of a combination of a polyol and a long-chain monofunctional fatty acid as a component of the polyester resin.
PCT International Application WO 95/19997 discloses a solventless coating composition based on a linear oligoester diol having a number average molecular weight in the range of from 275 to about 1,200. The linear oligoester diol of the patent is described as the reaction product of a linear dicarboxylic acid or ester and a linear diol. Thus, the above compositions have the disadvantage of the linear structured oligomeric esters described above.
Therefore, a need exists for liquid coating formulations without solvent or having extra high-solids content, without the disadvantages of having a low molecular weight, linear polyester vehicle. There is also a need for polyester resins which exhibit low viscosity.